1. Field of the Invention
This invention relates to slow release iron agricultural nutrients based on iron humate, and a method for their preparation. More particularly, this invention relates to a composition and a method for preparing the new slow releasing iron humate agricultural nutrient composition, which substantially completely releases its iron contents in soil solutions. The new method relates to the discovery that the coreaction of iron humate, a divalent metal oxide, an aqueous hydroxide of a monovalent Lewis acid, and a water soluble inorganic phosphate, produces hard, homogeneous, particles containing iron which is substantially insoluble in water, but soluble in agricultural soil solutions.
2. Description of Related Art
Iron is an essential nutrient in the growth of plants. It is usually absorbed by the roots of plants in ionic form from soil solutions. Iron deficiencies have been observed in many plant species where iron solubilities in the soil solution are insufficient to provide the plant iron requirements.
The amount of iron in the soil solution in relation to the amounts of other elements is as important in some instances as the absolute quantities of iron. Excess iron in the soil solution can cause plant deficiencies in other nutrients, such as manganese, copper, and molybdenum, even though ordinarily sufficient amounts of those nutrients are present in the soil.
Where fertilizers containing substantially water soluble iron are used around cement, brick, or ceramic walks, roads, floors, and walls, the surfaces are frequently stained with dark brown colored spots where the fertilizer granules contact the surfaces. The term substantially is used herein to mean mainly or more than one half.
The need for carbon in soils in which plants are grown has been well proved, and investigators have shown quantitatively the effectiveness of optimum ratios of carbon to nutrient, particularly nitrogen, ratios in the soil. Humates and humic acids are effective sources of soil carbon and additionally improve the condition of soils for growing plants.
Hjersted, in U.S. Pat. No. 5,213,692, disclosed a method for providing nutrients and organic matter to soil for use by vegetation, and a method for the preparation of iron humate by treatment of humate colored raw water, under controlled pH conditions, with an iron salt coagulant. Hjersted separated the precipitated iron humate product, concentrated and dried it. The iron humate recovered exhibited an iron solubility of less than 1 part per million parts of water. When he mixed the iron humate with nitrogen from urea or ammonia, he increased iron solubility in water to about 0.05 percent.
Soil is a heterogeneous, polydisperse system of solid, liquid and gaseous components. In this heterogeneous system, the soil solution acts as a medium by which chemical reactions between members of the different and the same soil phases are made possible, even when the reactants are not in direct contact. Nutrients to be effective in the growth of plants must be soluble in the soil solution which comprises soluble and insoluble cation exchange moieties, and organic and inorganic moieties. Soil solutions vary considerably in nature, and one solution may not be used to exactly duplicate each one's properties. However, neutral citrate solutions are used to indicate availability of nutrients to plants through the soil solutions, particularly in the cases of phosphate and iron nutrients.
To find iron nutrients with good availability of iron to plants, it has been a goal of researchers to find and produce iron nutrient compositions which have substantially complete solubilities in neutral aqueous citrate solution. The method to determine Citrate-Soluble Phosphorous in Fertilizers is defined in AOAC Official Methods of Analysis, 15th Edition, 1990 in Method 960.01. The same method may be used for Citrate-Soluble Iron using the AOAC Method 980.01 for iron instead of the phosphorous determination.
There exists an important need for an iron agricultural nutrient composition which is practically insoluble in water and substantially completely soluble in soil solution as indicated by solubility in aqueous neutral citrate solution, and for a method for effectively preparing such a composition.
In the preparation of agricultural nutrients, reactions involving acids and bases have been used. The term Lewis acid is used to define an ion, or molecule, that can combine with another ion, or molecule, by forming a covalent bond with two electrons from the second ion, or molecule. Hydrogen ion (proton) is the simplest substance which will do this, but many other ions such as ammonium ion, potassium, and sodium will also do this. Ammonium, sodium, and potassium hydroxides are hydroxides of Lewis acids.